Hitherto, there have been mainly produced two types of golf balls. The one is a solid golf ball, such as a two-piece golf ball or three-piece golf ball, and the other is a thread wound golf ball. The solid golf ball, when compared with the thread wound golf ball, has better durability and longer flight distance because of larger launch angle and smaller spin amount at the time of hitting. Therefore the solid golf ball is generally approved or employed by many golfers, mainly amateur golfers. In regard of enhancement of flight distance, the development of golf ball has been biased on the solid golf balls rather than the thread wound golf balls.
On the other hand, the solid golf ball exhibits hard and poor shot feel at the time of hitting. Solid golf balls having good shot feel have been recently proposed, but there is a drawback that rebound characteristics of the golf ball are degraded and the flight performance is reduced, because of softening of the ball.
In order to impart longer flight distance to the solid golf ball, a hollow core golf ball having a hollow core at its center has been proposed in Japanese Utility Model Publication No. 63354/1991. In the hollow golf ball, making the core hollow increased the moment of inertia, increases spin amount immediately after hitting, and reduces spin attenuation. It is proposed in Japanese Patent Kokai Publication No. 308709/1997 that a method of making the hollow golf ball comprises the steps of
preparing a semi-spherical half-shell formed from rubber by press-molding or injection-molding using a male plug mold, and PA1 bonding two such semi-spherical half-shells by vulcanizing in a mold. However, the hollow portion is deformed by thermal expansion of the rubber during vulcanizing, and it is not perfectly spherical, thereby the thickness of the core is uneven. In case of using a hollow core formed by bonding two vulcanized semi-spherical half-shells with an adhesive and the like, the hollow core is cut from the adhesive surface, and the durability is much degraded, because the impact force when hitting by a golf club is very large. PA1 bonding two vulcanized semi-spherical half-shells by placing the unvulcanized rubber composition between adhesion surfaces of the two vulcanized semi-spherical half-shells, and PA1 vulcanizing the unvulcanized rubber composition in a mold to form a hollow center, thereby providing a method of making a hollow golf ball having excellent durability, comprising a hollow portion that is perfect spherical shape.